Combine the egg yolks, 1/2 cup of the sugar, the pineapple (including the syrup), lemon juice, and buttermilk in the blender; whirl until blended (or beat together with rotary beater).
Soften the gelatin in water; stir over hot water until dissolved; then blend into buttermilk mixture.
Pour into a 2-quart container and freeze until firm around the outer edges of the container, about 1 1/2 hours.
Beat the egg whites until soft, moist peaks form; gradually beat in the remaining 1/4 cup sugar and continue beating until egg whites hold firm peaks.
Break the partially frozen sherbet into chunks, pour it into a chilled bowl and beat until fluffy, then fold into egg white mixture.
Return to freezer container and freeze until it is firm.